Sensors are often used in biomedical processes for converting biochemical reactions into electrical signals, for example, converting pH changes in a solution caused by biochemical reactions into electrical signals. Prior art sensors include ion-sensitive field-effect transistors (ISFETs), which can convert biochemical reactions into electrical signals with a certain degree of effectiveness. The use of ISFETs helps to achieve lower cost, higher throughput and a label free biomolecule detection. Further, ISFETs can be manufactured using mainstream CMOS technology. However, the sensitivity of conventional ISFETs is often defined by 59 mV/pH, which is the Nernst limit associated with an electrolyte and a site-binding surface. Such a sensitivity level may not be adequate for some applications such as human genome sequencing.
To date, various sensors with improved sensitivity have been developed for use in biomedical processes. Examples of such sensors include dual gate ISFETs having an additional back gate and sensors having dual transistors.
FIG. 1A shows a top view of a prior art sensor 100. The prior art sensor 100 is in the form of a coupled ISFET, and FIG. 1B shows a cross-sectional view of the prior art sensor 100. As shown in FIGS. 1A-1B, the prior art sensor 100 includes a first transistor 102 and a second transistor 104. The first transistor 102 includes a nano-plate 106 having a gate structure 108 in contact with a solution 110. The gate structure 108 includes a sensing element and a gate oxide layer. The second transistor 104 includes a nanowire 112 having a gate electrode 114 over a gate oxide layer 113. In use, the drain regions 116/118 of the first and second transistors 102, 104 are electrically coupled to a common voltage supply VDD, and the source regions 120/122 of the first and second transistors 102, 104 are electrically coupled to a current source 123 configured to provide a constant current flow ID through the sensor 100. The current flow ID splits into a first current flow through the first transistor 102 and a second current flow through the second transistor 104. The gate structure 108 of the first transistor 102 is configured to receive a voltage VG,1 based on a pH of the solution 110 and is further configured to control the first current flow through the first transistor 102 based on the voltage VG,1. When a pH of the solution 110 changes, the voltage VG,1 changes and the first current flow through the first transistor 102 changes. To maintain the combined current flow ID constant, the second current flow through the second transistor 104 changes to compensate for the change in the first current flow through the first transistor 102. This in turn changes the voltage VG,2 at the gate electrode 114 of the second transistor 104. The pH changes in the solution 110 are detected based on the amount of change in VG,2 to maintain the constant combined current flow ID.
Although prior art sensors, such as sensor 100, are capable of converting biochemical reactions into electrical signals by detecting pH changes in a solution, it would be useful to further improve the sensitivity of the sensors. One way to achieve higher sensitivities is to scale up one or more dimensions of the sensors, but this can cause an undesirable increase in the sensor sizes.